The invention relates to acoustic sensor devices with noise suppression to pick up the useful sound emitted by an object, and more particularly to sensor devices arranged on a noise-generating aggregate. Due to the noise level produced by the carrier aggregate for the sensor device at the sensing location, the acquisition range, i.e. the range for acoustic detecting and locating, of the sensor or the sensors of the device for measurement of the object sound is limited.
Passive noise suppression measures naturally have the disadvantage that not only the undesired noise is weakened, but also the useful sound from an object which is to be picked up is weakened. The active anti-noise systems described in the literature, for noise reduction, weaken or even compensate the entire sound field around the point under consideration, irrespective of its origin, by superposing upon it an anti-phase sound field. These anti-noise systems likewise reduce in an unfavourable manner both noise and useful sound because the sound from the object and the undesired noise are treated equally as far as suppression is concerned. Even if enough information is available either on the noise source or on the object emitting the useful sound to distinguish between useful signals and noise signals, for example by means of frequency-selective measures such as using adaptive digital filters, it cannot be avoided that the useful signal is weakened to a certain extent by the anti-phase sound, even though the noise can be weakened more selectively in this way.
To generate compensating oscillations or anti-phase signals, control circuits have previously been used to which the signal received by a sensor was supplied and which adjusted an anti-noise source on the basis of this signal. An example of this can be found in the German Patent No. DE 30 25 391 C2. In the device described in this patent a setting signal is supplied to the control circuit electro-acoustically, which setting signal represents the useful signal that varies over time and upon which the airborne noise signal coming from outside is superposed. The resulting oscillation, which is received by a microphone, is weighted by means of a linear filter and continuously compared with the setting signal. When a suitable frequency is chosen for the feedback path and for transmission of the useful signal, the interfering oscillation is successfully reduced and the useful signal is maintained in the resulting signal to a more or less satisfactory extent. However, this circuit does not work when both, the noise signal and the useful signal arrive through the air.
Another prior art device, German Published Patent Application No. DE 31 33 107 A1, does not solve the above problems, either. In the personal sound protection device proposed in this patent document, two microphones of different directional characteristics are directed toward one side. Because of the different directional characteristics, the noise and useful signal fractions differ in the electric signals supplied by the microphones if undesired noise and useful sound arrive from different directions. The two signals are supplied to a differential amplifier whose output signal is supplied to an output amplifier and represents the picked-up and selected useful signal, which is fed into an ear muff. Efficient suppression of undesired noise with maintenance of the maximum possible fraction of useful sound is possible only by means of different manual setting of the two microphone amplifiers.
The proposed control of the differential signal after low-pass filtering makes an efficient contribution only at specific interfering frequencies. This is effected by using a control circuit to return the voltage generated at the low-pass filter to the amplifier of the microphone which picks up mainly noise fractions, and by readjusting amplification until the low-pass voltage has decreased below a predetermined value. Apart from the fact that this measure is suited exclusively for low-frequency noise fractions and exclusively for higher-frequency useful sound fractions, it cannot be avoided that the sound fractions that are inevitably contained in the two signals are lost during subtraction. Even if the microphone amplifiers are set manually, this cannot be completely avoided. Such manual setting is suited only for personal sound protection by means of ear muffs, and not for a sensor device for detecting and locating useful sound.